Killing Is in My Veins
by TChallaInTheBathTub
Summary: Written for this prompt: Sherlock is a direct descendant of H. H. Holmes. One of the most famous and twisted serial killers. Dark!Sherlock, Dark!John. Please read the other warnings inside.


**Warnings:** Could be read as Dark!Sherlock and Dark!John. Discussion of murders, methods of killing and disposing of a body.

**Disclaimer: **I do not own _Sherlock_.

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><p>John was sitting on his chair with his laptop resting on his thighs. It was late and the flat was unusually quiet. Mrs. Hudson came back from having dinner with Mrs. Turner and her "married ones" almost an hour ago. John heard her waving them goodbye then retreating to her flat. She tried to drag John with her, but he was still feeling exhausted after the last case, so he declined the offer. As for the flatmate, he had no idea where Sherlock went off to, but he was glad for whatever caught the attention of his overly hyper friend. John does need some peace and quiet from time to time, which is something he doesn't get, often, living with the world's only Consulting Detective.<p>

Shaking those thoughts out of his mind, John went back to the article he was reading. He scrolled down taking all of the small details in and comparing them with what he already had stored in his mind. _Interesting_, he thought frowning at the screen.

The sound of a door slamming almost made him drop his precious laptop. Sherlock._ So much for a quiet night,_ he sighed rubbing his temple.

"John. are you awake?" Sherlock's voice came from downstairs and John can, clearly, see his flatmate taking off his coat and scarf while running up the stairs.

"Busy." John said.

"What are you doing?" Sherlock was standing right in front of him trying to peer at the screen from above.

"Reading." John scowled up at him half closing the screen.

Sherlock frowned at him then straighten up again. "I want tea."

"There is the kitchen. You can make it yourself."

"I don't want to."

"Well then, you can keep on wanting."

Sherlock stood for a moment before huffing and leaving to his room. John smiled, small wins are good.

John's eyes went back to the page and he went back to reading. Just as he was about to forget there was a ball of energy in the other room, Sherlock stormed into the room in his pajamas and dressing gown. John ignored him.

Sherlock was pacing the room and picking things up, looking at them, frowning at whatever he picked then throwing it back down. John tried not to yell at him to sit down, but his nerves betrayed him when Sherlock picked a book then let it fell on the floor.

"Alright. What is it?" John asked glaring at him.

"Isn't obvious?" Sherlock answered frowning at a pen.

"No it's not."

"John," his flatemate looked at John like the next words were meant to a child. "I. Am. Bored."

John took a deep breath fearing his next move, which involved his laptop flying from his hands to hit a head full of dark curls.

"You're always bored."

Sherlock sat heavily on the couch half pulling and half rubbing his hair. "I need something to do."

John sighed, he is used to this kind of behavior now. "Don't you have any new experiments you should be finishing?"

"No. I am still waiting for the tongue to decompose before I can continue the experiment."

"Do I want to know where you're keeping it?"

"Top of the fridge." Sherlock stretch on the couch.

"Alright. At least it's not inside this time."

"The fingers are still in there."

John shook his head. "I can't believe I am asking this. When are you going to experiment on those?"

"When I can get my hands on some toes. For some reason, Molly will not let me have any." Sherlock frowned.

"Ok." John left Sherlock to sulk on the couch and went back to his laptop.

He scrolled to where he stopped before Sherlock interrupted him. Again, he was just getting into the _"Sherlock is not here" _mood when the named person start talking.

"I think it's something to do with the eyes I took last week."

John knew he'll hate himself for responding. "What?" _Yes, he will._

"Molly. I took eyes from the morgue last week without asking. It could be the reason for her odd behavior."

"You always take stuff without asking."

"I know. That's the odd part of the whole situation, John. Try to keep up."

_Breath_, John told himself. "Could it be you insulting her in front of her new boyfriend _and_ insulting him at the same time?"

"The man was a dull, John. Too obvious and boring. He is obviously using her to get over his last fail relationship and will end it with her as soon as he realizes he is still in love with his ex."

"She likes him, Sherlock. And from what I saw, he likes her too. You can't deduce people's emotions."

"Yes I can. She can't like him."

"You can't decide who other people should or should not like."

"Why not?"

"Because it's none of your business what other people do with their personal lives."

Sherlock frowned at him then looked away, no doubt thinking of what John said. John observed him for a minute._ How can Sherlock think he can deduce other people's feeling when he can't see them on the person sitting near him?_ John shook his head looking back at his screen.

"John, _find_ me something." Sherlock's voice was too loud in the quiet room.

"It's late and if anyone dies, we won't hear about it until morning or if Lestrade text."

"After the last case, I doubt he will."

"I am not surprised."

A very short moment passed without a word then Sherlock's voice filled John's ears.

"I need a murder."

"For God's sake, Sherlock," John snapped. "Could you please stop thinking about murders and killers for 5 minutes."

Sherlock eyed him. Looking at him with narrowed eyes that made John shiver and look back down at his laptop.

"What are you reading?" Sherlock's voice was quieter this time.

"Reading about a serial killer," John could feel the smirk on Sherlock's face. He added fast before the detective says anything. "It's not what you think. I saw this when I was searching for the methods our copycat used on our last case when I saw this one, but I didn't have the time to read it before we found our killer. I got curious, that's all." He looked at Sherlock who raised an eyebrow at him.

"Who?" Sherlock said getting up from the couch to stand behind him.

"H.H. Holmes." John said looking up at his friend who was frowning. "You two might be related." John snickered.

Sherlock walked around him then sat down on his own chair facing John.

"We are."

"Excuse me?" John eyed him.

"He is one of my ancestors, John."

"Er... Really?"

"Yes. That surprises you?"

John thought about it, "No. No it doesn't" and Sherlock smirked.

"You can check The Holmes Family website if you want confirmation." Sherlock leaned back in the chair staring at him.

"Family website?" John smiled.

"Mother's idea. She wanted a place to keep track of everyone. Announcements to social rubbish and meetings. Ridicules idea, but of course Mycroft couldn't say no, so there is a Holmes family website." Sherlock was sulking again. John's smile was getting bigger.

"Alright, what's the name?"

"It's in your bookmarks," John's eyebrow rose. "I login sometimes with your laptop. You already know that."

"I know you consider my laptop as one of your properties, but I never thought you would actually check your family's website." John clicked on a link. "It says I need to login?"

"It's a private website. Use my name and my mobile number as the password."

"Your mobile number is your password?"

"Only for unimportant things."

"I see." The page opened. "Nice design." John remarked and Sherlock snorted. "Ah, the family tree." John clicked on the link.

John skimmed over the names until he found Sherlock's. He started reading backwards until he reached the name of Henry Howard Holmes and stopped. John's eyes widened a little, _Sherlock was a direct descendant of the famous serial killer._

"Wow." He said looking up at his friend who was smiling. "This is definitely interesting."

"Everyone got ancestors, John. That's not interesting."

"It is when one of them is a world famous serial killer."

Sherlock closed his eyes before his voice dropped saying the next words. "Scroll down and read the name of my father."

John did then looked back at Sherlock questionably. "What am I looking for exactly?"

"Having a famous serial killer as one of my ancestors is not as interesting," Sherlock opened his eyes, "as having a serial killer for a father."

_No, Sherlock didn't just say that._ "What?"

"Yes. I just said that, John."

John frowned reading the name in front of him again. It didn't ring any bells in his head. It is true John was not like Sherlock. He didn't know every single criminal or killer in history, which he was sure Sherlock had a special room for them in his memory palace, or even rooms! Anyway, John likes to think of himself as a normal human being who doesn't know everything about criminals, but still, he spent the last week reading about serial killers and this name did not come up anywhere.

"Mother made sure no one knew who he was." Sherlock's voice brought John back from his own memory palace.

John closed his laptop and put it on the floor giving Sherlock his full attention.

"I was 13 when the police came and took him away. I always knew something was off with him before, so I did my own investigation and I found out who he was."

Sherlock stopped talking and stared at him. John knew there will not be any explanation on how did Sherlock find out or why he didn't tell anyone or even when did he start feeling that his own father might be a murderer.

Sherlock was still leaning back on his chair with his eyes still fixed on John. But after a moment he leaned forward.

"John," Sherlock said licking his lips. "Do you believe criminals are born?"

"Born?" John tilted his head in confusion.

"Yes. Do you think criminal behavior is genetic?"

John leaned back in his chair. He doesn't know the answer to that. John did study some theories of criminal behaviors before. Although the theories about genetics being one of the main factors in criminal acts are very popular, he didn't believe it to be the only reason for anyone to snap and commit a crime.

"I don't believe having a history of crime in ones family makes them criminals. But I do believe it is one of the factors affecting this individual." John didn't know why was this conversation making him feel uncomfortable.

Sherlock nodded then closed his eyes leaning back into his chair again. "My father's first murder was before I was born. Mycroft was 4. His last murder was on the same day he was caught."

"How?" John heard himself asking.

"My mother," Sherlock's eyes were open. "She turned him in. She knew about what he'd been doing all along, but never said a word to anyone. I don't know what happened that night to make her call the police." John was looking closely at Sherlock's face. His friend didn't show any emotion telling the story. There were none at all.

"After the whole thing was over, she used her influence to cover it up. We moved to London and my father's name never came up again."

John didn't know how to respond to any of this. He was surprised when Sherlock started to talk about his family. Sherlock's private life was always a mystery to John. His flatmate never spoke of his past except of some stories from his university days, but never about his direct family. John suspects if Mycroft hadn't kidnapped him on the first night of meeting Sherlock, John would've never known about him. Well, at least until Mycroft decides to visit them to annoy his younger brother.

But something was bothering John. He eyed Sherlock who had his head dropped back and eyes closed again.

"Sherlock," John begun. "Why did you ask me if I believed in the genetic theory?"

Sherlock was still. Breathing deep like he was sleeping, John knew he wasn't. A minute gone by before the genius moved and his eyes locked with John's.

"Because I believe it is closely related."

John was silent for a moment. "You... Sherlock. You really believe that?"

"Yes." Why did that simple word send a chill down his spine? John realized Sherlock was totally serious.

"Well, I don't. I think it's a choice a person makes."

"Everything is a choice, John."

The doctor shook his head. _This feeling is unbearable_, he blow out a breath.

"I am going to make us some tea." He got up and went to the kitchen.

Putting the kettle on, John's mind went back to the first conversation he had with Sally Donovan.

_"One day just showing up won't be enough. One day, we'll be standing around a body and Sherlock Holmes will be the one that put it there."_

John almost smacked his own head._ This is ridiculous_, John knows his friend. It's true Sherlock could be a bit psychotic at times, but he was not a murderer. He grabbed the only two clean cups and poured the hot water in them. _Even if Sherlock's history is full of murderers, that doesn't mean he was one_, John picked up the cups and headed back to the living room.

John's laptop was on Sherlock's lap.

"Why is my laptop in your hands?"

"I am reading and mine is in my room."

"Your room is less than 50 steps away." John gave him his cup and sat down taking a sip from his own.

"Yours was less than 5 steps away." Sherlock took a sip without taking his eyes off the screen.

"Why do I even bother with you." John muttered into his tea. "What are you reading anyway?"

"H.H Holmes's methods," Sherlock's eyes were focus on the laptop he didn't notice John's expression,"you've been reading about him for at least 20 minutes before I came back. What's you impression?"

"He was insane."

"I meant his methods." Sherlock was smirking.

"Very smart, practical and nothing was left to chance," John frowned recalling what he had read. "The way he constructed his house was very, to be honest, admirable."

"A puzzle in a castle."

"That would make a good film," John found himself smiling which felt weird considering the topic of the conversation, "the use of acid and cremation were efficient in hiding the evidence and any trace of what he did."

"I agree on the cremation, but the acid would make a mess. In his case, the space and the construction of his house helped to conceal it. But in a smaller place, the acid would be a very bad idea. To do that, you'll need a big house or flat in a remote place with no nosy people around."

"221B will not work as a Murder Castle then." John said half joking.

Sherlock smirked, "I am afraid not."

"Storing the bodies in walls might be a good idea."

"No. It's not," Sherlock scowled at him. "If we do that, we'll need to change the layout of the whole place. I don't think Mrs. Hudson will go for it. You saw how she reacted when I shot the wall. I still don't understand what's the big deal."

John shook his head, "what about using 221C?"

"Will not work. We'll have to pay the rent for it, too. And what if we did use it without Mrs. Hudson knowledge, then someone rent it? No, too risky." Sherlock's frown was getting deeper, he was thinking too hard.

"How about going back to normal things," John said breaking the wheels in his friend's mind, "bury the body in a deserted place and leave."

"Someone might dig it up."

"That's why I said a deserted place."

"Every place is bound to be inhabited one day, John."

"weight it down with rocks and throw it into the Thames?"

"Someone might fish it out."

"Pigs?"

"John, we're in London." Sherlock blinked at him.

"Just trying to help." John was smiling despite himself.

Sherlock frowned, "cremation is the best solution of disposing a body."

After a short period of silence, John said "You've thought about this before, haven't you?"

"That surprises you?"

"No, nothing does anymore." Sherlock just grinned at him and looked back at the screen.

Although this whole conversation was not meant to be like they were actually talking about killing some poor person, John still couldn't shake the feeling that was ripping into his chest.

John was looking down at the black liquid in his hands with his brain racing with thoughts. What if what Donovan had said was right? Will Sherlock snap one day and kill someone? John didn't think his friend was capable of such a thing, on the other hand, he was.

He took another sip and his eyes went to look at the figure sitting across from him. John had killed people before. He was a doctor, but war doesn't know the difference between an army doctor or an army soldier. He killed a lot of people before he was shipped back home.

John killed people after the war. The cabbie was the first, but not the last. Each time there was a reason behind each kill. War, protect innocent people, protect Sherlock and sometimes self-defense. But deep down, John knows he is capable of murder without blinking.

"You're thinking too much."

"I thought that was your area not mine."

"My thoughts are always precise. Yours are all over the place." John snorts drinking his cold tea.

"We both are capable of doing it, you know," Sherlock's voice rang in the room. John didn't say anything. His eyes were staring at Sherlock,

"it is a choice we both made before, even if the circumstances were essential in that choice."

"So, you're saying," John licked his lips, "you might murder somebody one day?"

"We both might, John." Sherlock looked up at him.

Silence fell on them when Sherlock broke eye contact to resume his reading. John got up taking both cups with him to the kitchen. Sighing, he looked around the mess. He should clean this before Mrs. Hudson sees the place. But he couldn't bring himself to do anything other than leaving the cups in the sink and going back to the room, cleaning could wait one more day.

"I did thought about it." Sherlock's voice was low and barely audible in the quiet room.

It took John a second to understand what Sherlock meant. "Murdering people?" He asked sitting back down.

"Yes."

The answer didn't really surprise John. After all, this is Sherlock. John was sure his flatmate thought and tried every thing he could think of. "I know you did."

"I might one day," the laptop was still on the detective's lap although forgotten._ One day,_ John didn't know if that was a promise or something else.

"I know." He said without breaking eye contact. What else could he say? He believes in his friend. He also believes everyone is capable of doing anything they set their minds on, and Sherlock always gets what he wants no matter what the cost may be.

"And you'll help me cover everything." From Sherlock's tone, John knew that was not a question. It was a statement.

A moment of silence then.

"I know." John said looking directly into Sherlock's bright eyes. Although he wanted to deny it and yell No. But John knew, _God help him_, he would do anything for Sherlock, including helping him cover up a crime.

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><p>Sometime in the future. <p>

_I need your help._

_S_

_What's wrong?_

_J_

_I've just murdered someone._

_S_

_Where are you?_

_J_

_Home._

_S_

_I am on my way._

_J_

John took a deep breath then walked out of the surgery calmly. _This is going to be a long day,_ he thought getting into a cab.

"221B Baker Street, please."

And so it begins.


End file.
